Processes commonly used to fabricate acrylic sheets, such as thermoforming, cementing, machining, line bending, buffing, flame polishing and screen printing, can cause stress to the acrylic.
Annealing the acrylic allows fabricators to strengthen the acrylic sheet and reduce or eliminate the fabrication stress through a process of controlled heating and then cooling.
An annealing oven
Let’s take a closer look at this interesting process, using The Handbook of Acrylics as a resource.
Acrylic Can be Heat Treated for Superior Clarity & Strength
Annealing the acrylic completes the polymerization of the curing process and increases resistance to crazing caused by either stress or chemical attack.
Induced stresses caused by machining or bonding are subsequently eliminated by annealing.
The annealing process requires a carefully controlled heat application and cool down. Annealing is performed in large, specially designed, process ovens in the factory and, when necessary, in custom-built portable ovens on the job site.
Special care must be exercised when annealing a panel that is scheduled for bonding to another panel. Unless thoroughly annealed, edges of the panel will craze when wetted by the acrylic monomer cement during bonding.
This crazing occurs when the acrylic surface is under high enough residual tensile stress to have the monomer cement trigger the appearance
of stress crazing.
A craze, by definition, is a lens-shaped damage zone containing induced microvoids within a highly-oriented polymer chain. Within the crazed zone, voids comprise 40-60% of the area’s volume.
These voids allow an increase in material cross-section without lateral contraction because the lateral contraction is prohibited, at least in thick specimens, by the elastic constraint of the surrounding or adjacent undeformed polymer.
The craze propagates transversely to the principal stress vector, thereby maximizing the spreading stress at the tip of the craze. Crazes contain material which has a lower refractive index and density than the bulk of the resin.
They are characterized as being quite narrow and are often difficult to see. In laminated transparencies, premature crazing is a particularly serious problem because the crazes tend to form on the outer surface of the acrylic plies.
The threshold value of residual stress which triggers the appearance of stress crazing depends on the length of time that it takes for the monomer cement to polymerize (i.e., turn from liquid into a solid). Since most of the monomer cements, depending on their proprietary composition, require anywhere from 1 to 12 hours to polymerize, the residual stress on the
edge of the acrylic panel must be below the threshold value at which crazing will be triggered during 12 hours of exposure to the liquid monomer.
This threshold value of residual stress that initiates crazing on an acrylic surface after a 12-hour exposure to acrylic monomer is 175 psi. Thus, the annealing process must be effective enough to reduce any tensile residual stresses on the acrylic panels prior to bonding to < 100 psi level.
But even in cast panels not slated for bonding with acrylic monomer cement, the residual stress should be minimized by thorough annealing so that crazing of the viewing surfaces is not triggered by cleaning with inappropriate solvents, for example, alcohol.
Annealing is a Time-at-Temperature Phenomenon
It is also dependent upon the thickness and grade of the acrylic: the thicker the sheet, the longer the annealing time.
Annealing temperatures are normally above the heat-distortion temperature of the material, i.e., from 200 to 230°F (93 to 110°C), and the time is that period necessary to bring the material to thermal equilibrium.
Cooling is a necessary part of the operation and must be controlled to prevent part warpage caused by residual stress formation. The cooling period is normally considered to be complete when the midplane temperature is below the glass transition temperature of the material.
Acrylic can be bolted or riveted to substructures, but normally edge attachment materials are first bonded to the acrylic to provide a way to distribute the fastener load. Acrylic is “notch” sensitive and differential thermal expansion must be considered.
In summary, proper annealing of formed and finished acrylic plastics is the most effective preventive measure against crazing in unstretched acrylic plastics.
It consists of prolonged heating at an elevated temperature followed by slow cooling. The internal stresses set up during fabrication and machining are reduced or eliminated by this treatment. It also results in greater dimensional stability and resistance to crazing.
To obtain these benefits, it is necessary that the procedure be performed
after all other fabrication procedures, including polishing, are completed.
Annealing of machined windows must be performed at least once and preferably twice. The first process should be done on rough-cut discs at sufficiently high temperature 230° (110°C) to anneal them thoroughly and to stabilize them dimensionally (diameter shrinks < 2.2 percent and thickness increases < 4 percent).
The second annealing should occur at low temperature 185°F (85°C) only when all machining and polishing operations have been completed.
Because shrinkage took place during the first annealing process, no dimensional changes occur during final annealing of the finished product.